Enhancement of superconducting transition temperature in FeSe electric-double-layer transistor with multivalent ionic liquids

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Enhancement of superconducting transition temperature in FeSe electric-double-layer transistor with multivalent ionic liquids

Tomoki Miyakawa, Junichi Shiogai, Sunao Shimizu, Michio Matsumoto, Yukihiro Ito, Takayuki Harada, Kohei Fujiwara, Tsutomu Nojima, Yoshimitsu Itoh, Takuzo Aida, Yoshihiro Iwasa, and Atsushi Tsukazaki

Phys. Rev. Materials 2, 031801(R) – Published 26 March 2018

Abstract

We report on an enhancement of the superconducting transition temperature $(T_{c})$ of the FeSe-based electric-double-layer transistor (FeSe-EDLT) by applying the multivalent oligomeric ionic liquids (ILs). The IL composed of dimeric cation (divalent IL) enables a large amount of charge accumulation on the surface of the FeSe ultrathin film, resulting in inducing electron-rich conduction even in a rather thick 10 nm FeSe channel. The onset $T_{c}$ in FeSe-EDLT with the divalent IL is enhanced to be approaching about 50 K at the thin limit, which is about 7 K higher than that in EDLT with conventional monovalent ILs. The enhancement of $T_{c}$ is a pronounced effect of the application of the divalent IL, in addition to the large capacitance, supposing preferable interface formation of ILs driven by geometric and/or Coulombic effect. The present finding strongly indicates that multivalent ILs are powerful tools for controlling and improving physical properties of materials.

Received 11 January 2018
Revised 20 February 2018

DOI:https://doi.org/10.1103/PhysRevMaterials.2.031801

Physics Subject Headings (PhySH)

Electrochemistry Superconductivity

High-temperature superconductors Liquid-solid interfaces

Condensed Matter, Materials & Applied PhysicsInterdisciplinary Physics

Authors & Affiliations

Tomoki Miyakawa¹, Junichi Shiogai^1,*, Sunao Shimizu², Michio Matsumoto^3,†, Yukihiro Ito¹, Takayuki Harada¹, Kohei Fujiwara¹, Tsutomu Nojima¹, Yoshimitsu Itoh³, Takuzo Aida^2,3, Yoshihiro Iwasa^2,4, and Atsushi Tsukazaki¹

¹Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
²RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan
³Department of Chemistry and Biotechnology, The University of Tokyo, Tokyo 113-8656, Japan
⁴Quantum-Phase Electronics Center (QPEC) and Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japan

^*Author to whom correspondence should be addressed: junichi.shiogai@imr.tohoku.ac.jp
^†Present address: Department of Chemistry, Northwestern University, Evanston, IL 60208, USA.

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Vol. 2, Iss. 3 — March 2018

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Images

Figure 1
(a) Schematic cross section of FeSe-EDLT in this study. Application of a positive $V_{G}$ accumulates IL cations on the FeSe surface, inducing Helmholtz layer at the interface. (b)–(d) The molecular structures of (b) monovalent DEME-TFSI, (c) divalent IL2-TFSI, and (d) tetravalent IL4-TFSI. (e) $I_{G} − T$ curves for each device at $V_{G} = 5 V$ . The arrows indicate the $T$ -sweep direction. The black lines are fits to the data above the threshold temperature upon increase of $I_{G}$ . (f) The x-ray diffraction pattern of a pristine 16.6-nm-thick FeSe film around FeSe (001) diffraction peak. (g) That after some etching cycles (temporal integral $I_{G} \sim 34.4 mC)$ using IL2-TFSI.
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Figure 2
(a)–(c) Hall resistance $(R_{yx})$ versus magnetic field ( $B$ ) at $T = 50 K$ under $V_{G} = 5 V$ for FeSe-EDLTs at (a) $d \sim 11 nm$ , (b) $d \sim 6 nm$ , and (c) $d \sim 1 nm$ . Red, blue, and green solid lines represent the data taken with IL2-, DEME-, and IL4-TFSI, respectively. For clarity, $R_{yx}$ were symmetrized with respect to $B$ . (d) Thickness dependences of the Hall coefficients obtained by the slope of $R_{yx} − B$ curves for EDLTs with IL2- (red circles), DEME- (blue triangles), and IL4-TFSI (green squares). (e)–(j) Schematics for possible charge distribution in FeSe-EDLTs with IL2-TFSI at each thickness condition: (e) thick, (f) intermediate, and (g) thin. (h)–(j) Those for FeSe-EDLTs with DEME-TFSI. The interface of ILs/FeSe locates at the left sides of these panels. Electron and hole distributions are separately depicted by solid yellow and blue, because the band structure of FeSe is semimetallic with two bands at different $k$ direction. Broken lines indicate Fermi energy for both electron and hole bands. At the thick condition in FeSe-EDLT with IL2-TFSI [Fig. 2], the band bending occurs in the whole thickness region due to the large capacitance of divalent IL.
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Figure 3
(a), (b) $T$ dependence of $R_{s} / R_{s}^{60 K}$ measured at various $d$ with (a) IL2-TFSI and (b) IL4-TFSI. For clarity, the $R_{s}$ is normalized against the value at 60 K $(R_{s}^{60 K})$ . The $d$ was calibrated with etching rates on the assumption that the decrease in $d$ is proportional to the temporal integration of $I_{G}$ . (c) The $T$ dependence of the normalized $R_{s}$ in the vicinity of transition under $V_{G} = 5 V$ for FeSe-EDLTs with DEME- (blue), IL2- (red), and IL4-TFSI (green) at $d \sim 1 nm$ . The inset shows the same set of $R_{s} − T$ curves at $d \sim 11 nm$ . Thickness error bars are within 1 nm. The $T_{c}^{on}$ is defined as the temperature at the intersection of two fitted lines [see broken lines in (c)] as indicated by the black arrow.
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Figure 4
$T_{c}^{on}$ as a function of $R_{H}$ measured at $T = 50 K$ for IL2- (red circles), DEME- (blue triangles), and IL4-TFSI (green squares). The gray line is a guideline of the $T_{c}^{on} - R_{H}$ plot extracted from Ref. [12]. The data were taken under the gate voltage of $V_{G} = 5 V$ .
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